A total of 220 shoulders will be included in the study (55 shoulders in each of the four groups). Each shoulder will be assessed preoperatively, at 3 weeks, 3 months, one year, two years and five years. The primary end-point will be range of motion at one year. Additional end-points will include dislocation, stress fractures of the acromion, metal ion levels in peripheral blood, neurovascular complications, radiographic notching, implant mechanical failure and revision surgery.